US20200140263A1 - Reduced mems cavity gap - Google Patents

Abstract

Provided herein is a method including forming a MEMS cap. A cavity is formed in the MEMS cap wafer, and a bond material is deposited on the MEMS cap wafer, wherein the bond material lines the cavity after the depositing. The MEMS cap wafer is bonded to a MEMS device wafer, wherein the bond material forms a bond between the MEMS cap wafer and the MEMS device wafer. A MEMS device is formed in the MEMS device wafer. The bond material is removed from the cavity.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/754,069 filed Nov. 1, 2018, entitled “METHOD AND SYSTEM TO INCREASE CAVITY PRESSURE AND DECREASE DIELECTRIC CHARGING.”
BACKGROUND
• MEMS (“micro-electro-mechanical systems”) are a class of devices that are fabricated using semiconductor-like processes and exhibit mechanical characteristics. For example, MEMS devices may include the ability to move or deform. In many cases, but not always, MEMS interact with electrical signals. A MEMS device may refer to a semiconductor device that is implemented as a micro-electro-mechanical system. A MEMS device includes mechanical elements and may optionally include electronics (e.g. electronics for sensing). MEMS devices include but are not limited to, for example, gyroscopes, accelerometers, magnetometers, pressure sensors, etc. As technology advances, it is desirable to reduce the size of the MEMS devices, thereby resulting in die size reduction.
SUMMARY
• Provided herein is a method including forming a MEMS cap wafer. A cavity is formed in the MEMS cap wafer, and a bond material is deposited on the MEMS cap wafer, wherein the bond material lines the cavity after the depositing. The MEMS cap wafer is bonded to a MEMS device wafer, wherein the bond material forms a bond between the MEMS cap wafer and the MEMS device wafer. A MEMS device is formed in the MEMS device wafer. The bond material is removed from the cavity. These and other features and advantages will be apparent from a reading of the following detailed description.
BRIEF DESCRIPTION OF DRAWINGS
• FIG. 1 shows a cap wafer including cap cavities according to one aspect of the present embodiments.
• FIG. 2 shows an oxide formed on the first side and the cap cavity according to one aspect of the present embodiments.
• FIG. 3 shows the cap wafer fusion bonded to a MEMS device wafer including standoffs according to one aspect of the present embodiments.
• FIG. 4 shows the formation of the MEMS device according to one aspect of the present embodiments.
• FIG. 5 shows the removal of the bond in the cap cavity according to one aspect of the present embodiments.
• FIG. 6 shows eutectic bonding of the MEMS wafer to a CMOS wafer according to one aspect of the present embodiments.
• FIG. 7 shows an exemplary flow diagram for removing the bond material from the cavity according to one aspect of the present embodiments.
DESCRIPTION
• Before various embodiments are described in greater detail, it should be understood that the embodiments are not limiting, as elements in such embodiments may vary. It should likewise be understood that a particular embodiment described and/or illustrated herein has elements which may be readily separated from the particular embodiment and optionally combined with any of several other embodiments or substituted for elements in any of several other embodiments described herein.
• It should also be understood that the terminology used herein is for the purpose of describing the certain concepts, and the terminology is not intended to be limiting. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood in the art to which the embodiments pertain.
• Unless indicated otherwise, ordinal numbers (e.g., first, second, third, etc.) are used to distinguish or identify different elements or steps in a group of elements or steps, and do not supply a serial or numerical limitation on the elements or steps of the embodiments thereof. For example, “first,” “second,” and “third” elements or steps need not necessarily appear in that order, and the embodiments thereof need not necessarily be limited to three elements or steps. It should also be understood that, unless indicated otherwise, any labels such as “left,” “right,” “front,” “back,” “top,” “middle,” “bottom,” “beside,” “forward,” “reverse,” “overlying,” “underlying,” “up,” “down,” or other similar terms such as “upper,” “lower,” “above,” “below,” “under,” “between,” “over,” “vertical,” “horizontal,” “proximal,” “distal,” and the like are used for convenience and are not intended to imply, for example, any particular fixed location, orientation, or direction. Instead, such labels are used to reflect, for example, relative location, orientation, or directions. It should also be understood that the singular forms of “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise.
• Terms such as “over,” “overlying,” “above,” “under,” etc. are understood to refer to elements that may be in direct contact or may have other elements in-between. For example, two layers may be in overlying contact, wherein one layer is over another layer and the two layers physically contact. In another example, two layers may be separated by one or more layers, wherein a first layer is over a second layer and one or more intermediate layers are between the first and second layers, such that the first and second layers do not physically contact.
• A MEMS device includes mechanical elements and may optionally include electronics (e.g. electronics for sensing). MEMS devices include but are not limited to, for example, gyroscopes, accelerometers, magnetometers, microphones, pressure sensors, etc. As technology advances, it is desirable to reduce the size of the MEMS devices, thereby resulting in die size reduction. According to embodiments described herein, the dielectric oxide in the MEMS cap cavity above the MEMS device is removed using, for example, vapor hydrofluoric acid after the MEMS actuator etch. Removal of the dielectric oxide allows reduction of the cavity gap above the MEMS device, resulting in many advantages. For example, a reduced cavity gap above a MEMS accelerometer increases the accelerometer damping by reducing the cavity volume and increasing pressure. In addition, reducing the cavity gap provides better thermal and electrostatic balance between the MEMS cap cavity and a CMOS (“complementary metal-oxide semiconductor”) cavity, as well as rejection of common mode noise sources.
• Referring now toFIG. 1, a cap wafer including cap cavities is shown according to one aspect of the present embodiments. Acap wafer102 is formed using any suitable fabrication method. Thecap wafer102 may be, for example, a silicon wafer, however it is understood that embodiments are not limited to silicon wafers. Thecap wafer102 may also be referred to as a MEMS cap wafer and includes afirst side104 and asecond side106. A cap cavity108 (e.g. MEMS cap cavity) is formed in thefirst side104 of thecap wafer102. Thecap cavity108 may be formed by any suitable fabrication method (e.g. etching, cutting, laser ablation, etc.). It is understood that in various embodiments, one ormore cap cavities108 may be formed in thecap wafer102.
• Referring now toFIG. 2, an oxide formation on the first side and the cap cavity is shown according to one aspect of the present embodiments. Alining210 is formed over thefirst side104 using any suitable fabrication method (e.g. deposition, growth, sputtering, etc.). Thelining210 is a bond material, for example a thermal oxide growth (e.g. silicon dioxide) or chemical vapor deposition (CVD) oxide. Thelining210 covers thefirst side104 and thecap cavity108. For example, in various embodiments, thelining210 is deposited on the on thecap wafer102. After thelining210 is deposited, it lines thefirst side104 and thecap cavity108.
• Referring now toFIG. 3, the cap wafer fusion bonded to a MEMS device wafer including standoffs is shown according to one aspect of the present embodiments. AMEMS device wafer312 is bonded to thecap wafer102 with abond314 formed from the lining210 (FIG. 2). For example, thecap wafer102 may be fusion bonded to theMEMS device wafer312, thereby forming a silicon oxide silicon bond. In various embodiments, theMEMS device wafer312 is a structural layer including silicon that will be formed into various MEMS devices (e.g. accelerometer, gyroscope, etc.).
• Together, thecap wafer102 and theMEMS device wafer312 may be referred to as aMEMS wafer316. Therefore, theMEMS wafer316 includes thecap wafer102 and theMEMS device wafer312. Thecap wafer102 and theMEMS device wafer312 define acavity gap318 in thecap cavity108. Thecavity gap318 extends from the top of thecap cavity108 in thecap wafer102 down to the surface of theMEMS device wafer312.
• According to embodiments described herein, thecavity gap318 is smaller than previous cavity gaps. In addition, in some embodiments thecap cavity108 may be sized relative to a CMOS cavity630 (seeFIG. 6). For example, the size of thecap cavity108 may be smaller than, equal to, or larger than the size of theCMOS cavity630.
• In some embodiments,standoffs320 are patterned on theMEMS device wafer312, and a metal321 (e.g. germanium) used for eutectic bonding (seeFIG. 6) is deposited on thestandoffs320. Thestandoffs320 define the vertical clearance between the structural layer (e.g. the MEMS device wafer312) and a CMOS wafer (not shown but seeFIG. 6). Thestandoffs320 may also provide electrical contact between theMEMS wafer316 and the CMOS wafer.
• Referring now toFIG. 4, the formation of the MEMS device is shown according to one aspect of the present embodiments. AMEMS device420 has been formed in theMEMS device wafer312 using any suitable fabrication method, for example etching thedevice wafer312 using deep reactive ion etching (“DRIE”). Fabrication methods may include, but are not limited to, etching, cutting, laser ablation, deposition, growth, sputtering, etc. TheMEMS device420 may be any MEMS device, including, but not limited to, gyroscopes, accelerometers, microphones, and pressure sensors. Therefore, theMEMS device wafer312 includes theMEMS device420 surrounded by thecap cavity108.
• As illustrated, thecap cavity108 overlies theMEMS device420. Thus, thecap cavity108 and theMEMS device420 define thecavity gap318 overlying theMEMS device420. At this stage of manufacture, thebond314 lines thecap cavity108. In various embodiments, thebond314 may be a fusion bond, and the bond material may include a fusion bond oxide.
• Referring now toFIG. 5, removal of the bond in the cap cavity is shown according to one aspect of the present embodiments. Thebond314 has been removed from the inside of thecap cavity108. As such, thecap cavity108 is no longer lined with the bond material (e.g. the bond314). In various embodiments, the bond material is removed through theMEMS device420, and thecap wafer102 remains bonded to theMEMS device wafer312 after the removal of the bond material.
• The bond material may be removed from thecap cavity108 by selective etching of thebond314. For example, the selective etching may include introducing (seeFIG. 5 arrow522) vapor HF (“hydrofluoric acid”), liquid HF, BOE (“buffered oxide etchant”), or ME (“reactive ion etch”) into thecap cavity108 through the MEMS device402 to remove the bond material. It is understood that in the embodiments described herein, removal of the bond material from thecap cavity108 occurs after the bonding of theMEMS device wafer312 to thecap wafer102, as described above in reference toFIG. 3. Furthermore, it is understood that in various embodiments all of the of the bond material may be removed from thecap cavity108, or some of the bond material may be removed from portions of thecap cavity108.
• Referring now toFIG. 6, eutectic bonding of the MEMS wafer to a CMOS wafer is shown according to one aspect of the present embodiments. TheMEMS wafer316 is eutecticly bonded to aCMOS wafer624 witheutectic bonds626. In various embodiments, theeutectic bonds626 are aluminum-germanium (AlGe) bonds. In other embodiments, theeutectic bonds626 can be formed by tin-copper, tin-aluminum, gold-germanium, gold-tin, or gold-indium. Theeutectic bonds626 bond thestandoffs320 tobond pads628 on theCMOS wafer624. As such, theeutectic bonds626 provide electrical connections between theMEMS wafer316 and theCMOS wafer624. TheMEMS device wafer312, thestandoffs320, and theCMOS wafer624 define aCMOS cavity630 below theMEMS device420. In some embodiments,eutectic bonds626 form a hermetic seal around theMEMS device420. Thus, the pressure and/or gas composition may be set within thecap cavity108 and theCMOS cavity630. For example, the pressure may be set to greater than, equal to, or lesser than atmospheric pressure. It is understood that theCMOS wafer624 is an integrated circuit (“IC”) substrate with one or more electrical circuits.
• As previously discussed, reducing the size of thecap cavity108 results in asmaller cavity gap318. This is advantages because, for example, the reducedcavity gap318 above a MEMS accelerometer (e.g. the MEMS device420) increases the accelerometer damping by reducing the volume and increasing pressure of thecap cavity108. In addition, reducing thecavity gap318 provides better thermal and electrostatic balance between the cap cavity108 (e.g. MEMS cap cavity) and theCMOS cavity630, as well as rejection of common mode noise sources.
• However, thebond314 lining the cap cavity108 (seeFIG. 4) causes problems with the reduction of thecap cavity108. For example, contact between theMEMS device420 and thebond314 lining thecap cavity108 can result in charging, which can drift over the lifetime of the product and cause performance degradation. Therefore as previously discussed, according to embodiments described herein, thebond314 lining thecap cavity108 is removed after fusion bonding of theMEMS device wafer312 to thecap wafer102. As a result, the size reduction of thecap cavity108 is advantageous instead of detrimental (as described above).
• Furthermore, it is important that removal of thebond314 lining thecap cavity108 occurs after fusion bonding, in order to ensure that the lining210 (seeFIG. 2) is not damaged. For example, attempts to remove the lining210 from thecap cavity108 prior to fusion bonding produce damage to the lining210 in other areas. For example, even minor damage caused to thelining210, using processes such as selective etching or masking prior to fusion bonding, can result in poor fusion bonding of theMEMS device wafer312 to thecap wafer102.
• FIG. 7 shows an exemplary flow diagram for removing the bond material from the cavity according to one aspect of the present embodiments. Atblock702, a MEMS cap wafer is formed. At ablock704, a cavity is formed in the MEMS cap wafer. At ablock706, a bond material is deposited on the MEMS cap wafer, wherein the bond material lines the cavity after the depositing. At ablock708, the MEMS cap wafer is bonded to a MEMS device wafer, wherein the bond material forms a bond between the MEMS cap wafer and the MEMS device wafer. At ablock710, a MEMS device is formed in the MEMS device wafer. In various embodiments, the MEMS device may be a gyroscope or accelerometer.
• At ablock712, the bond material is removed from the cavity. In some embodiments, the removing is after the bonding. In further embodiments, the bond material is removed through the MEMS device, and the MEMS cap wafer remains bonded to the MEMS device wafer after the removing. In various embodiments the bond material may be selectively etched. The selective etching may include introducing any one of vapor hydrofluoric acid, liquid HF, BOE, or RIE into the cavity through the MEMS device.
• In some embodiments, a MEMS wafer includes the MEMS cap wafer and the MEMS device wafer, and the MEMS wafer is eutecticly bonded to a CMOS wafer. In various embodiments the bond material includes an oxide.
• While the embodiments have been described and/or illustrated by means of particular examples, and while these embodiments and/or examples have been described in considerable detail, it is not the intention of the Applicants to restrict or in any way limit the scope of the embodiments to such detail. Additional adaptations and/or modifications of the embodiments may readily appear, and, in its broader aspects, the embodiments may encompass these adaptations and/or modifications. Accordingly, departures may be made from the foregoing embodiments and/or examples without departing from the scope of the concepts described herein. The implementations described above and other implementations are within the scope of the following claims.

Claims (14)

What is claimed is:

1. A method comprising:

forming a micro-electro-mechanical system (“MEMS”) cap wafer;

forming a cavity in the MEMS cap wafer;

depositing a bond material on the MEMS cap wafer, wherein the bond material lines the cavity after the depositing;

bonding the MEMS cap wafer to a MEMS device wafer, wherein the bond material forms a bond between the MEMS cap wafer and the MEMS device wafer;

forming a MEMS device in the MEMS device wafer; and

removing the bond material from the cavity.

2. The method ofclaim 1, wherein the bond material is removed through the MEMS device, and wherein further the MEMS cap wafer remains bonded to the MEMS device wafer after the removing.

3. The method ofclaim 1, further comprising selective etching the bond material.

4. The method ofclaim 3, wherein the selective etching includes introducing any one of vapor HF (“hydrofluoric acid”), liquid HF, BOE (“buffered oxide etchant), or ME (“reactive ion etch) into the cavity through the MEMS device.

5. The method ofclaim 1, wherein the removing includes removing with vapor HF, liquid HF, BOE, or RIE.

6. The method ofclaim 1, wherein a MEMS wafer includes the MEMS cap wafer and the MEMS device wafer, and further comprising eutecticly bonding the MEMS wafer to a CMOS wafer.

7. The method ofclaim 1, wherein the bond material includes an oxide.

8. The method ofclaim 1, wherein the removing is after the bonding.

9. The method ofclaim 1, wherein the MEMS device is a gyroscope, accelerometer, microphone, or pressure sensor.

10. The method ofclaim 1, wherein forming the MEMS device includes etching the device wafer using deep reactive ion etching (“DRIE”).

11. An apparatus comprising:

a cap cavity in a micro-electro-mechanical system (“MEMS”) cap wafer, the cap cavity overlying a MEMS device;

a MEMS wafer including the MEMS cap wafer and a MEMS device wafer, the MEMS device wafer including the MEMS device; and

a bond connecting the MEMS cap wafer to the MEMS device wafer, wherein

the bond includes a material, and

the material has been removed from portions of the cavity.

12. The apparatus ofclaim 11, further comprising

a complementary metal-oxide semiconductor (“CMOS”) wafer; and

a eutectic bond electrically connecting the CMOS wafer to the MEMS wafer.

13. The apparatus ofclaim 11, wherein the bond is a fusion bond.

14. The apparatus ofclaim 11, wherein the material is a dielectric fusion bond oxide.

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